In various industrial fields there is frequently the necessity of drying the surfaces of finished pieces which in the preceding machining steps, due to different reasons, had been immerged in water.
It often happens too that, besides the removal of water, there is also the need to remove powders (such as residual abrasive powders) or deposits of machining scraps not thoroughly removed previously. The reasons which impose such necessity may be of merely aesthetical nature, such as in the field of goldsmith's wares, silver wares, spectacles and in the galvanic field.
In some cases, however, deposits of any kind which are not removed can adversely affect the performances of finished pieces, such as in the case of the electric, electronic, nuclear industry, as well as in the high-precision mechanical industry. In these cases, the machining scraps, the water, the natural salts contained in the water, the salts solubilized by water during the preceding treatments, may adhere, also in the form of microparticles, to the pieces and penetrate into inaccessible areas and they are removable only by means of techniques comprising the use of solvents having a relatively high density and being properly additioned.
The presence of all such foreign matters could cause short circuits, corrosions, decompositions to corrosive products, or could accelerate corrosion or abrasion phenomena or in any case in some way alter the calibration characteristics of high-precision proportioning valves, of electronic cards, of printed circuits which, as is known, are more and more often calibrated with the highest accuracy by employing the laser technology.
The problem of drying has already been differently envisaged.
British Pat. No. 1,285,509, for example, described the use of water-immiscible solvents additioned with salts obtained from unsaturated aliphatic carboxylic acids and diamines in which one of the two nitrogen atoms is substituted by an alkyl or by an alkylene. Another patent, French Pat. No. 2,217,045, claims the use of a composition consisting of a fluorine-containing solvent, with a solubility parameter, as is defined in J. Appl. Chem. 3, 71 (1953), lower than 8, and of a surfactant: the mixture is characterized in that the H.sub.2 O/solvent-interface tension does not exceed 6 dynes/cm and in that the employed surfactant has a water solubilization coefficient, measured in CCLF.sub.2 --CCL.sub.2 F, lower than 750 ppm.
A third patent, G.B. Pat. No. 1,428,530, besides claiming, as surfactants, diamides of unsaturated carboxylic acids and N-monosubstituted diamines, describes a machine suitable for such type of washing and of drying. It essentially consists of a tank in which the wet pieces are immersed: the removed water forms little drops which quickly collect on the surface, float on the organic solvent and are easily separated by decantation. The pieces are then rinsed with a pure solvent to remove the surfactant still present on the surfaces.
From an examination of the above-cited patents it appears that, in order to have satisfactory performances, at least 0.1% of surfactant is required. Such amount is relatively high and it would be desirable to find compositions in which the amount of additive is sensibly lower.
This would result in two main advantages:
(1) a reduction in the surfactant amount which, during the drying step, adheres to the washed pieces: this would facilitate the rinsing step in the pure solvent and would reduce the yet small amount of residual surfactant, thus ensuring a higher quality of the treated article;
(2) a reduction of the environmental pollution caused by possible dispersions of surfactants in the wastes.